The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer, and the like.
An image forming apparatus which employs an intermediary transfer belt has long been known. An image forming method employing an intermediary transfer belt is effective for full-color image forming apparatuses or multicolor image forming apparatuses for synthetically obtaining full-color images or multicolor images through an image formation process, in which a plurality of temporary images different in color formed sequentially in accordance with full-color image formation data or multicolor image formation data are sequentially transferred in layers onto an intermediary transfer medium to synthesize full-color images or multicolor images, respectively.
FIG. 5 shows the general structure of a typical image forming apparatus employing an intermediary transfer belt in accordance with conventional arts. This image forming apparatus is a color image forming apparatus (copying machine, laser beam printer, or the like) having an intermediary transfer belt 120. The material for this intermediary transfer belt 120 is an elastic substance, the electrical resistance of which is in the mid range.
This image forming apparatus comprises an electrophotographic photoconductive member 101 (which hereinafter will be referred to as photoconductive drum) in the form of a drum. This photoconductive drum 101 is rotationally driven in the direction indicated by an arrow mark at a predetermined peripheral velocity (process speed).
As the photoconductive drum 101 is rotationally driven, it is uniformly charged by a primary charging device 102 to predetermined polarity and potential level, and is exposed to a beam of light 103 projected by an exposing means while being modulated with image formation data. As a result, an electrostatic latent image corresponding to one of the color components (for example, yellow color component) of an intended color image is formed on the photoconductive drum 101.
Next, the electrostatic latent image is developed into a visual image, that is, a yellow toner image, by first developing device 141 (yellow color component developing device), in the developing station. During this process of developing the electrostatic latent image corresponding to the yellow color component, second to fourth developing devices 2-4, that is, a magenta color component developing device 142, a cyan color component developing device 143, and a black color component developing device 144, are kept inactive; they do not act on the photoconductive drum 101. Therefore, the yellow toner image is not affected by the second to fourth developing devices 142-144. The first to fourth developing devices 141-144 are mounted in a supporting member 140 rotatable about its center axle, being enabled to be sequentially moved to the development station, in which they oppose the photoconductive drum 101 as they are moved into the development station.
The intermediary transfer belt 120 is stretched around rollers 161 and 162, and a primary transfer roller 125, being positioned so that it is placed in contact with the peripheral surface of the photoconductive drum 101 at a predetermined contact location. It is rotationally driven at the same peripheral velocity as that of the photoconductive drum 101 in such a direction that its peripheral surface moves in the same direction as that of the photoconductive drum 101, at the contact location. At the contact location, a primary transfer roller 125 is disposed on the inward side of the loop of the intermediary transfer belt 120, so that a primary transfer bias can be applied to the intermediary transfer belt 120 from a bias power source 129 through the primary transfer roller 125. The primary transfer bias is opposite in polarity to the toner, and its potential level is within the range of +100 V-2 kV.
While the yellow toner image on the photoconductive drum 101 is passed through the contact area, or recording paper nipping portion (which hereinafter will be simply referred to as nip), between the photoconductive drum 101 and intermediary transfer belt 120, it is continually transferred onto the outwardly facing surface of the intermediary transfer belt 120, with reference to the belt loop, by the primary transfer electric field formed by the primary transfer bias applied to the intermediary transfer belt 120 through the primary transfer roller 125.
After the transfer of the yellow toner image, that is, the toner image corresponding to the first color component, onto the intermediary transfer belt 120, the primary transfer residual toner particles, that is, the toner particles remaining on the peripheral surface of the photoconductive drum 101 after the primary transfer of the yellow toner image, are removed by a cleaning apparatus 113; the peripheral surface of the photoconductive drum 101 is cleaned by the cleaning apparatus 113. Thereafter, the cleaned portion of the peripheral surface of the photoconductive drum 101 is again subjected to the above described image forming process which begins with the primary charging of the photoconductive drum 101, to form a magenta toner image, or the toner image corresponding to the second color component, on the photoconductive drum 101, and transfer the magenta toner image onto the intermediary transfer belt 120 in such a manner that the magenta image is placed in layers on the yellow toner image. This process is repeated two more times, to sequentially form a cyan toner image, or the toner image corresponding to the third color component, and a black toner image, or the toner image corresponding to the fourth color component, on the photoconductive drum 101, and sequentially transfer them in layers onto the preceding two toner images on the intermediary transfer belt 120. As a result, a synthetic full-color image reflecting the intended color image is effected on the intermediary transfer belt 120.
The roller 164, which supports the intermediary transfer belt 120, doubles as a roller which opposes a secondary transfer roller 163 located outside the loop of the intermediary transfer belt 120. The secondary transfer roller 163 is enabled to be moved so that it can be pressed against the roller 164 with the interposition of the intermediary transfer belt 120, or can be moved away from the intermediary transfer belt 120 and roller 164. To the secondary transfer roller 163, a secondary transfer bias is applied from a bias power source 128. Further, the secondary transfer roller 163 is enabled to be kept away from the intermediary transfer belt 120 during the primary transfer of the toner images corresponding to the first-fourth color components of the intended image.
Immediately before the four color toner images transferred in layers on the intermediary transfer belt 120 reach the secondary transfer station due to the rotation of the intermediary transfer belt 120, the application of the secondary transfer bias to the secondary transfer roller 163 from the bias power source 128 is started, and at the same time, the secondary transfer roller 163 is placed in contact with the intermediary transfer belt 120. Meanwhile a transfer medium P as recording medium (paper, resinous sheet, and the like) is sent into the recording medium path by a sheet feeding roller pair 111 with a predetermined timing, and is delivered, being guided by a guide 110, to the contact area between the secondary transfer roller 163 and intermediary transfer belt 120.
While the transfer medium P is passed through the contact area, or the recording paper nipping portion, between the transfer roller 163 and intermediary transfer belt 120, the four color toner images, different in color, on the intermediary transfer belt 120 are continually transferred together onto the transfer medium P by the secondary transfer electric field formed by the secondary transfer bias applied to the intermediary transfer belt 120 from the secondary transfer roller 163. After the transfer of the four color toner images onto the transfer medium P, the transfer medium P is introduced into a fixing device 115, in which the four color toners are fixed (melted, mixed, and permanently adhered to the transfer medium P), by the heat and pressure applied by the fixing apparatus 115, effecting a permanent full-color print.
The secondary transfer residual toner particles, that is, the toner particles remaining on the peripheral surface of the intermediary transfer belt 120 after the secondary transfer, are charged by a belt cleaner 108 to the polarity opposite to that of the photoconductive drum 101. The belt cleaner 108 is a roller located outside the loop of the intermediary transfer belt 120, being enabled to be placed in contact with, or moved away from, the intermediary transfer belt 120. More specifically, the secondary transfer residual toner particles are charged to the predetermined polarity as a cleaning bias with the predetermined polarity is applied to the belt cleaner, that is, the roller 108, from a bias power source 126 through a grounded electrically conductive roller 107 as the opposing electrode, disposed within the loop of the intermediary transfer belt 120, while the roller 108 is kept in contact with the outward surface of the intermediary transfer belt 120. In this embodiment, the photoconductive drum 101 is negatively charged. Thus, the secondary transfer residual toner particles are charged to the positive polarity. The belt cleaner 108 is enabled to be kept away from the intermediary transfer belt 120 during the primary transfer of the first to third toner images corresponding to the first to third color components, one for one.
After being charged to the polarity opposite to that of the photoconductive drum 101, the secondary transfer residual toner particles on the intermediary transfer belt 120 are electrostatically attracted onto the photoconductive drum 101, in the contact area between the intermediary transfer belt 120 and photoconductive drum 101, as well as the adjacencies of the contact area; they are removed from the intermediary transfer belt 120.
In the case of a color image forming apparatus, such as the one disclosed in Japanese Laid-open Patent Application 63-301960, which employs a transfer drum, a color image is obtained by transferring a plurality of toner images different in color onto a transfer medium held on the peripheral surface of a transfer drum, directly from the photoconductive drum. Thus, this type of color image forming apparatus requires a recording medium controlling means for holding a recording medium to the transfer drum (for example, recording medium may be held to transfer drum by gripper, glue, suction, or may be given a curvature matching that of peripheral surface of transfer drum). In comparison, the color image forming apparatus in this embodiment employs the above described intermediary transfer belt, eliminating the need for the above described recording medium controlling means. Further, the provision of the intermediary transfer belt enables the color image forming apparatus in this embodiment to form satisfactory color images regardless of the thickness, width, and length of a recording medium; in other words, it enables the apparatus to form satisfactory color images on an envelope, a post card, a label, or the like, in addition to ordinary recording paper, although the thickness of a recording medium should be within the range of 40 g/m2 (thin paper)-200 g/cm2 (cardboard).
Because of the above described advantage, a color image forming apparatus employing an intermediary transfer belt is widely used in the form of a color copying machine, a color printer, and the like.
Currently, the desire for faster speed, higher quality, and longer service life is very strong even in the field of image forming apparatus employing an intermediary transfer belt. In the field of color image forming apparatus, efforts have been made, in particular, for higher image quality. For example, in recent years, the number of users who use glossy paper to obtain high quality images which give the impression of an image created by silver salt photography has been increasing. Also, some users began to form an image on both sides of glossy paper to make a book. However, because glossy paper is generally high in electrical resistance, the usage of glossy paper in the low humidity environment has been creating the problem of the electrical discharge occurring in the secondary transfer station, resulting in a defective image. This problem was more prevalent in the case of double-sided printing. As a method for preventing the image defects traceable to the electrical discharge which occurs as a printing paper enters the secondary transfer station, the method disclosed in Japanese Laid-open Patent Application 9-90780 is well known.
This method is effective to prevent electrical discharge from occurring on the entrance side of the secondary transfer station. However, it does not mention the electrical discharge which occurs at the exit side of the secondary transfer station.
In spite of the superiority of an intermediary transfer belt in terms of the conformity to recording paper, even an image forming apparatus employing an intermediary transfer belt suffers from its own problems. One of the two significant problems of such an image forming apparatus is the image defects (image defects of a first type) traceable to the electrical discharge which occurs when a transfer medium enters the secondary transfer station, and the other is the image defects (image defects of a second type, which look like bird s foot prints) traceable to the electrical discharge which occurs when the recording medium exits from the secondary transfer station. In spite of this fact, the measure for preventing the image defect of the second type is not proposed in the method disclosed in Japanese Laid-open Patent Application.
The primary object of the present invention is to prevent the image defects which occur in the second transfer station, that is, the contact areas between the intermediary transfer belt and photoconductive member, so that it becomes possible to provide an image forming apparatus capable of forming high quality images on a wide variety of recording medium.
According to the primary aspect of the present invention, an image forming apparatus comprises: an image bearing belt for bearing an image; a supporting roller for supporting said image bearing belt; a transfer roller which forms a transfer medium nipping portion against said image bearing member; and a regulating member for regulating the direction in which a transfer medium is discharged from said transfer medium nipping portion, wherein a single or plurality of images on said image bearing belt are transferred by the application of voltage to said transfer roller onto a transferring medium being moved; said transfer medium nipping portion has a first area which is not backed up by said supporting roller, and across which said image bearing member and transfer roller make contact with each other, and a second area which is backed up by said supporting roller, and across which said image bearing belt and transfer roller make contact with each other, and wherein the first area of said transfer medium nipping portion is on the upstream side of the second area of said transfer medium nipping portion in terms of the moving direction of the transfer medium.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.